Niobium-based alloys with improved structural properties for high temperature structural applications

ABSTRACT

A niobium-based refractory alloy. The inclusion of measurable, controlled amounts of interstitial nitrogen improves one or more structural properties of the alloy. Components made with the alloy have higher strengths, especially at elevated temperatures, while ductility and oxidation resistance are not adversely affected. In particular, components for use in ground-based and air-based gas turbine engine hot sections can utilize the alloy in order to realize improvements in one or more of service life, operational efficiency and design simplification.

This application claims the benefit of U.S. Provisional Application Ser. No. 63/333,649 that was filed on Apr. 22, 2012.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The subject matter of the present disclosure was made with government support under Contract No. FA8650-21-F-5271 that was awarded by the United States Air Force. The government has certain rights in such subject matter.

TECHNICAL FIELD

The present disclosure relates generally to Nb-based refractory alloys that have improved structural properties compared to current Nb-based refractory alloys, and more particularly to Nb-based refractory alloys that through the inclusion of controlled amounts of elemental nitrogen exhibit such improved properties for components made therewith.

BACKGROUND

Niobium (Nb)-based refractory alloys are used in high-temperature structural applications, such as in the hot (that is to say, turbine and exhaust duct) sections of a gas turbine engine. Traditionally, elemental impurities such as carbon (C), nitrogen (N) and oxygen (O) are generally unavoidable in commercial operations and, when present, tend to form in the atomic interstices of the alloy. Such impurities are deemed to be a minor nuisance in small quantities and—in higher quantities—deleterious to one or more figures of merit, such as various mechanical and structural properties, of a resulting alloy or structural component being formed with such alloy. Accordingly, their inclusion is kept to a practical minimum when creating Nb-based refractory alloys.

SUMMARY

The authors of the present disclosure have discovered that including measurable amounts of N can significantly improve numerous structural properties for an Nb-based alloy, as well as for components made with such alloy. Examples of such improved properties include increasing strength without sacrificing ductility at room temperatures and above, particularly at the high temperature exposures that components made with such alloys are expected to endure. Gas turbine engine combustors, turbines and exhausts are components that routinely encounter high temperatures that could benefit from the strength and ductility improvements, as permitting higher temperature operation without an inordinate amount of parasitic cooling can significantly increase overall thermal efficiency, while the additional strengths permit simpler component designs that in turn can help reduce weight the latter of which is particularly beneficial in aircraft-related applications.

According to an aspect of the present disclosure, a niobium-molybdenum-titanium (Nb—Mo—Ti) alloy is disclosed. The alloy includes, by weight, about 10 percent to about 34 percent Mo, about 2 percent to about 20 percent Ti, up to about 30 percent hafnium (Hf), up to about 10 percent aluminum (Al), up to about 20 percent chromium (Cr), up to about 15 percent tantalum (Ta), up to about 15 percent tungsten (W), up to about 10 percent zirconium (Zr), about 0.1 percent to about 3.0 percent N, at least one elemental alloy addition selected from the group consisting of C, O and mixtures thereof and a balance of Nb. Although not being bound by theory, the authors of the present disclosure believe that the beneficial properties fir the N levels depicted herein arise out of their evaluation of Nb—N, Mo—N and Ti—N binary phase diagrams, where the volume fraction of the stable nitride phase reaches approximately 50 percent by volume at about 3.0 percent (by weight) of N.

According to another aspect of the present disclosure, a component made from an Nb—Mo—Ti alloy with enhanced structural properties is disclosed. The alloy that makes up the component includes, by weight, about 10 percent to about 34 percent Mo, about 2 percent to about 20 percent Ti, up to about 30 percent hafnium (Hf), up to about 10 percent aluminum (Al), up to about 20 percent chromium (Cr), up to about 15 percent tantalum (Ta), up to about 15 percent tungsten (W), up to about 10 percent zirconium (Zr), about 0.1 percent to about 3.0 percent N, at least one elemental alloy addition selected from the group consisting of C, O and mixtures thereof and a balance of Nb.

According to another aspect of the present disclosure, a gas turbine component made from an Nb—Mo—Ti alloy is disclosed. The component includes a substrate and a coating disposed over at least a portion of the substrate, where the substrate is made from an alloy that includes, by weight, about 10 percent to about 34 percent Mo, about 2 percent to about 20 percent Ti, up to about 30 percent hafnium (Hf), up to about 10 percent aluminum (Al), up to about 20 percent chromium (Cr), up to about 15 percent tantalum (Ta), up to about 15 percent tungsten (W), up to about 10 percent zirconium (Zr), about 0.1 percent to about 3.0 percent N, at least one elemental alloy addition selected from the group consisting of C, O and mixtures thereof and a balance of Nb.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative in nature and not intended to limit the subject matter defined by the claims. The following detailed description of the illustrative embodiments can be understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:

FIGS. 1A and 1B depict measured compositions of certain baseline Nb—Mo—Ti alloys containing low levels of N in weight and atomic percent, respectively;

FIG. 2 depicts strength properties of the alloys of FIG. 1 at 25° C., 1000° C. and 1300° C.;

FIG. 3 depicts phase analysis micrographs of one of the alloys (R-2) of FIGS. 1A and 1B;

FIG. 4 depicts phase analysis micrographs of one of the alloys (R-6) of FIGS. 1A and 1B;

FIG. 5 depicts phase analysis micrographs of one of the alloys (R-8) of FIGS. 1A and 1B;

FIG. 6 depicts phase analysis micrographs of one of the alloys (R-10) of FIGS. 1A and 1B;

FIGS. 7A and 7B depict measured compositions of certain Nb—Mo—Ti alloys containing increased levels N according to the present disclosure, the measure compositions being in weight and atomic percent, respectively;

FIG. 8 depicts yield properties of some of the alloys of FIGS. 7A and 7B;

FIGS. 9A through 9F show stress-strain comparisons of some of the alloys represented by those of FIGS. 1A, 1B, 7A and 7B where the test was conducted at an elevated temperature;

FIGS. 10A through 10F show stress-strain comparisons of alloys represented by those of FIGS. FIGS. 1A, 1B, 7A and 7B where the test was conducted at room temperature;

FIG. 11 depicts a phase analysis for some of the alloy samples of FIGS. 7A and 7B;

FIG. 12 depicts micrographs and phase analysis for one of the alloys (RN-1) of FIGS. 7A and 7B;

FIG. 13 depicts micrographs and phase analysis for another of the alloys (RN-9) of FIGS. 7A and 7B; and

FIG. 14 depicts a gas turbine engine airfoil that may be made with one or more of the alloys according to the present disclosure.

DETAILED DESCRIPTION

According to certain aspects of the present disclosure, a technical problem relates to how to use Nb-based alloys in components that are designed for high temperature applications in such a way that the components don't suffer adverse decreases in various structural properties that could either adversely affect component service life or lead to a sub-optimal design of the component. In this regard, aspects of the present disclosure provide a technical solution that utilizes small, intentional additions of N in order to improve one or more figures of merit relating to these structural properties. The authors of the present disclosure have discovered that while excess elemental N in such an alloy is commonly believed to be deleterious to certain structural properties of the alloy, including controlled amounts of N produces surprisingly beneficial improvements in such properties. The technical problem and the technical solution disclosed herein are particularly relevant in gas turbine engine hot sections.

Referring first to FIGS. 1A and 1B, compositions of twelve different baseline Nb—Mo—Ti alloy samples are shown where the N level (as well as that of other interstitial elements) is kept to very low levels. The samples were produced by vacuum arc melting of high purity (>99.9%) elements, and then subjected to hot isostatic pressing (HIP) at various temperatures. Unless otherwise noted, all component or composition levels are in reference to the active portion of that component or composition, and are exclusive of impurities, for example, residual solvents or by-products, which may be present in commercially available sources of such components or compositions. Alloys R-1, R-2, R-3, R-4, R-6 and R-12 were HIP′d at 1400° C. under hydrostatic pressure of 207 MPa for 3 hours, while alloys R-5, R-7, R-8, R-9 and R-10 were HIP′d at 1200° C., also under a hydrostatic pressure of 207 MPa for 3 hours.

Referring next to FIG. 2 , compression properties of the alloys of FIGS. 1A and 1B are shown. The crystal structure of the alloys are as follows: Alloys R-1 through R-4, R-6, R-12 and R-13 are single-phase BCC structures, Alloys R-5, R-9 and R-10 have a BCC matrix with Laves C15 secondary phase, Alloy R-7 has a BCC matrix with Laves C14 secondary phase while Alloy R-8 has a BCC matrix with Laves C14 and sigma secondary phases. It is noted that single-phase refractory complex concentrated alloys (RCCAs) have a balance of strength and ductility, while multiphase RCCAs are brittle at room temperature and those containing C14 tend to have inferior strength at high temperatures (for example, 1300° C.). The mechanical properties were measured using compression testing at 25° C. (air) and 1300° C. (vacuum of more than 10-5 Torr). The compression samples had rectangular cross-sections with dimensions of 4.5 mm (width), 4.5 mm (thickness) and 7.6 mm (height). In addition, the various phases (including various secondary phase precipitates) associated with some of these alloys are shown. The crystal structure of the alloys is as follows: Alloys R-1 through R-4, R-6, R-12 and R-13 are single-phase BCC structures, Alloys R-5, R-9 and R-10 have a BCC phase as a matrix and cubic Laves phases as precipitates, Alloy R-7 has a BCC phase as a matrix and hexagonal Laves phases as precipitates while Alloy R-8 has three phases made up of a BCC matrix phase and a hexagonal Laves phase with a sigma precipitate secondary phase.

Referring next to FIG. 3 , a phase analysis of the single-phase baseline Alloy R-2 is shown. The alloy essentially has a single-phase BCC structure with an average grain size of 103 μm.

Referring next to FIG. 4 , a phase analysis of the single-phase baseline Alloy R-6 is shown. The alloy is essentially single-phase BCC structure. A very small fraction (less than 0.01% by volume) of Hf-rich precipitates is detected at the grain boundaries. The average grain size of this alloy is 214 μm.

Referring next to FIG. 5 , a phase analysis of the multiphase baseline Alloy R-8 is shown. X-ray diffraction and scanning electron microscopy (SEM) analyses indicate that the alloy consists of a BCC matrix phase and precipitates of a hexagonal Laves and tetragonal Sigma phases.

Referring next to FIG. 6 , a phase analysis of the multiphase baseline Alloy R-10 is shown. According to X-ray diffraction, SEM and backscatter electron diffraction analyses, this alloy consists of the BCC matrix phase and cubic Laves phase precipitates. Small amounts of a hexagonal Laves phase are also identified in this alloy.

Referring next to FIGS. 7A and 7B, compositions of the Nb—Mo—Ti alloy samples that have been modified as disclosed herein to include measurable (that is to say, higher) levels of N. As with the samples of FIGS. 1A and 1B, these samples were produced by vacuum arc melting of high purity (>99.9%) elements and subject to the same commercial-level of impurities, residual solvents or the like. It will be appreciated that the presence of any such impurities—as well as other compounds, components, elements or related materials, including those that are intentionally introduced—within the disclosed compositions are permissible so long as they don't materially change the characteristics of the alloys, compositions or components as claimed herein. Within the present context, such materially changed characteristics are those that would prevent, adversely affect or reduce the designability or usability of the claimed alloys, compositions or components for achieving the structural properties (such as strength or ductility, as well as fracture toughness and resistance to corrosion, oxidation, wear and thermal creep), service life or other intended purposes as discussed herein. Relatedly, the introduction of other compounds, components, elements or related materials (such as those including those that are otherwise already present in the disclosed alloys) in quantities greater than or less than the amounts disclosed herein that would materially change the characteristics of the resulting alloy are deemed impermissible.

As can be seen, N levels of between roughly 2 and 8 atomic percent (roughly 0.3 and 1.4 weight percent) are shown, compared to the low levels in conventional Nb-based alloys of between almost 0 and 0.22 atomic percent (almost 0 and 0.03 weight percent). While not being bound by theory, the authors of the present disclosure believe that the crystal structure of the FIGS. 7A and 7B alloys contain substantially the same phases as their FIGS. 1A and 1B counterparts, in addition to having one or more of titanium nitride (Ti_(x)N_(y))-based and hafnium nitride (Hf_(x)N_(y))-based precipitates.

Referring next to FIG. 8 , compression strength properties of some of the alloys of FIGS. 7A and 7B are shown where the crystal structure of these alloys are as follows: Alloys RN-1 through RN-4, RN-6, RN-12 and RN-13 have a BCC matrix with one or more of Nb, Ti, Hf and N-type precipitates, Alloy RN-5 has a BCC matrix with Laves C15 and one or more of Nb, Ti, Hf and N-containing secondary phases, Alloys RN-9 and RN-10 have a BCC matrix with Laves C15 and Nb, Ti and N-containing secondary phases, Alloy RN-7 has a BCC matrix with Laves C14 and one or more of Nb, Ti, Hf and N-containing secondary phases while Alloy RN-8 has a BCC matrix with Laves C14, sigma and one or more of Nb, Ti and N-containing secondary phases. As with the samples of FIGS. 1A and 1B, the mechanical properties of these samples were measured using compression testing at 25° C. (air) and 1300° C. (vacuum of less than 10-5 Torr). The compression samples had rectangular cross-sections with dimensions of 4.5 mm (width), 4.5 mm (thickness) and 7.6 mm (height).

Referring next to FIGS. 9A through 9F, compressive tests were conducted at elevated temperature, specifically at 1300° C. Stress-strain comparisons between some of the alloys of FIGS. 1A, 1B, 3A and 3B are shown. As can be seen, both yield stress and peak stress values show significant improvement in the alloys where the N was higher.

Referring next to FIGS. 10A through 10F, compressive tests were conducted at room temperature, specifically at 25° C. As with the high temperature tests of FIGS. 9A through 9F, both yield stress and peak stress values show significant improvement in the alloys where the N was higher. The alloys that have enhanced levels of N exhibit a larger strain hardening effect after yielding; this in turn may be a factor leading to larger true peak stress values. Significantly, all of the alloys, which are single-phase BCC structures at low concentrations of N, demonstrated adequate ductility after alloying with N, and none fractured after a 50% height reduction.

Although not shown, preliminary test results for some of the alloys of FIGS. 7A and 7B exhibit—in addition to the improvements in strengths depicted in FIGS. 9A through 9F and 10A through 10F—improvements in ductility.

Referring next to FIG. 11 , a phase analysis of some of the alloys containing increased amount of N is shown, displaying phases and phase lattice parameters observed in the alloys.

Referring next to FIGS. 12 and 13 , micrographs and phase analyses of the multi-phase (BCC and FCC) Alloys RN-1 RN-9 are shown.

Referring next to FIG. 14 , a gas turbine engine airfoil is shown that may be made with one or more of the alloys disclosed herein. It will be appreciated that one or more additional coatings (such as an oxidation coating, thermal barrier coating (TBC) or environmental resistant bond coating) may employ one or more of the improved structural property alloys disclosed herein. It will also be appreciated that the depiction of a turbine blade is by example, not limitation, and that other components that are exposed to high temperature conditions during their service life may also be made with eh alloys disclosed herein, and that all such other components are deemed to be within the scope of the present disclosure.

Within the present disclosure, all of the composition percentages and ratios are calculated by weight unless otherwise indicated. In addition, all of the percentages and ratios are calculated based on the total composition unless otherwise indicated. Likewise, every maximum numerical limitation disclosed herein is understood to include every lower numerical limitation, as if such lower numerical limitations were expressly written. In addition, every minimum numerical limitation disclosed herein is understood to include every higher numerical limitation, as if such higher numerical limitations were expressly written. Every numerical range given throughout this specification is understood to include every narrower numerical range that falls within such broader numerical range, as if such narrower numerical ranges were all expressly written.

Within the present disclosure, one or more of the following claims may utilize the term “wherein” as a transitional phrase. For the purposes of defining features discussed in the present disclosure, this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising” and its variants that do not preclude the possibility of additional acts or structures.

Within the present disclosure, terms such as “preferably”, “generally” and “typically” are not utilized to limit the scope of the claims or to imply that certain features are critical, essential, or even important to the disclosed structures or functions. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the disclosed subject matter. Likewise, it is noted that the terms “substantially” and “approximately” and their variants are utilized to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement or other representation. As such, use of these terms represents the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

Within the present disclosure, the use of the prepositional phrase “at least one of” is deemed to be an open-ended expression that has both conjunctive and disjunctive attributes. For example, a claim that states “at least one of A, B and C” (where A, B and C are definite or indefinite articles that are the referents of the prepositional phrase) means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together.

Within the present disclosure, the following claims are not intended to be interpreted based on 35 USC 112(f) unless and until such claim limitations expressly use the phrase “means for” or “steps for” followed by a statement of function void of further structure. Moreover, the corresponding structures, materials, acts and equivalents of all means or step plus function elements in the claims are intended to include any structure, material or act for performing the function in combination with other claimed elements as specifically claimed.

Within the present disclosure, the singular forms “a,” “an” and “the” include plural references unless the context clearly dictates otherwise. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (for example, it includes at least the degree of error associated with the measurement of the particular quantity). The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4” also discloses the range “from 2 to 4.” The terms “comprise(s),” “include(s),” “having,” “has,” “can,” “contain(s),” and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that do not preclude the possibility of additional acts or structures. The present disclosure also contemplates other embodiments “comprising,” “consisting of” and “consisting essentially of,” the embodiments or elements presented herein, whether explicitly set forth or not.

For the recitation of numeric ranges herein, each intervening number therebetween with the same degree of precision is explicitly contemplated. For example, for a range of 6 through 9, the numbers 7 and 8 are contemplated in addition to 6 and 9, and for the range 6.0 through 7.0, the numbers 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9 and 7.0 are explicitly contemplated.

The present description is for purposes of illustration and is not intended to be exhaustive or limited. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the present disclosure. Aspects of the present disclosure were chosen and described in order to best explain the principles and practical applications, and to enable others of ordinary skill in the art to understand the subject matter contained herein for various embodiments with various modifications as are suited to the particular use contemplated.

Unless otherwise defined, all technical and scientific terms used herein that relate to materials and their processing have the same meaning as commonly understood by one of ordinary skill in the art. In case of conflict, the present document, including definitions, will control. For example, the term “yield strength” as used herein refers to the stress level at which plastic deformation begins.

While particular embodiments have been illustrated and described herein, it should be understood that various other changes and modifications may be made without departing from the spirit and scope of the claimed subject matter. Moreover, although various aspects of the claimed subject matter have been described herein, such aspects need not be utilized in combination. It is therefore intended that the appended claims cover all such changes and modifications that are within the scope of the claimed subject matter. 

What is claimed is:
 1. A niobium-molybdenum-titanium alloy comprising, by weight: about 10 percent to about 34 percent molybdenum; about 2 percent to about 20 percent titanium; up to about 30 percent hafnium; up to about 10 percent aluminum; up to about 20 percent chromium; up to about 15 percent tantalum; up to about 15 percent tungsten; up to about 10 percent zirconium; about 0.1 percent to about 3.0 percent nitrogen; at least one elemental alloy addition selected from the group consisting of carbon, oxygen and mixtures thereof; and a balance of niobium.
 2. The alloy of claim 1, wherein the molybdenum comprises between about 14 percent and about 34 percent by weight.
 3. The alloy of claim 1, wherein the titanium comprises between about 2.7 percent and about 14.4 percent by weight.
 4. The alloy of claim 1, wherein the hafnium comprises between about 0 percent and about 29 percent by weight.
 5. The alloy of claim 1, wherein the aluminum comprises between about 0 percent and about 6.8 percent by weight.
 6. The alloy of claim 1, wherein the chromium comprises between about 0 percent and about 18.3 percent by weight.
 7. The alloy of claim 1, wherein the tantalum comprises between about 0 percent and about 0.1 percent by weight.
 8. The alloy of claim 1, wherein the tungsten comprises between about 0 percent and about 6.1 percent by weight.
 9. The alloy of claim 1, wherein the zirconium comprises between about 0 percent and about 5 percent by weight.
 10. The alloy of claim 1, wherein the nitrogen comprises between about 0.3 percent and about 1.4 percent by weight.
 11. The alloy of claim 10, wherein the nitrogen comprises between about 0.5 percent and about 1.2 percent by weight.
 12. The alloy of claim 11 wherein the nitrogen comprises between about 0.6 percent and about 1.3 percent by weight.
 13. The alloy of claim 12, wherein the nitrogen comprises between about 0.7 percent and about 1.2 percent by weight.
 14. The alloy of claim 13, wherein the nitrogen comprises between about 0.8 percent and about 1.1 percent by weight.
 15. The alloy of claim 1, wherein at least a portion of the titanium is present as a titanium nitride-based secondary phase precipitate.
 16. The alloy of claim 1, wherein at least a portion of the hafnium is present as a hafnium nitride-based secondary phase precipitate.
 17. The alloy of claim 1, wherein at least a portion of the titanium is present as a titanium nitride-based secondary phase precipitate and at least a portion of the hafnium is present as a hafnium nitride-based secondary phase precipitate.
 18. A component with enhanced structural properties, the component made from a niobium-molybdenum-titanium alloy comprising, by weight: about 10 percent to about 34 percent molybdenum; about 2 percent to about 20 percent titanium; up to about 30 percent hafnium; up to about 10 percent aluminum; up to about 20 percent chromium; up to about 15 percent tantalum; up to about 15 percent tungsten; up to about 10 percent zirconium; about 0.1 percent to about 3.0 percent nitrogen; at least one elemental alloy addition selected from the group consisting of carbon, oxygen and mixtures thereof; and a balance of niobium such that the structural properties that are selected from a group consisting of yield strength, true peak stress and true fracture stress, exhibit increases their respective figures of merit over a niobium-molybdenum-titanium alloy that comprises nitrogen in an amount greater or less than the percent recited herein.
 19. A gas turbine engine component comprising: a substrate comprising a niobium-molybdenum-titanium alloy comprising, by weight: about 10 percent to about 34 percent molybdenum; about 2 percent to about 20 percent titanium; up to about 30 percent hafnium; up to about 10 percent aluminum; up to about 20 percent chromium; up to about 15 percent tantalum; up to about 15 percent tungsten; up to about 10 percent zirconium; about 0.1 percent to about 3.0 percent nitrogen; at least one elemental alloy addition selected from the group consisting of carbon, oxygen and mixtures thereof; and a balance of niobium; and a coating disposed over at least a portion of the substrate.
 20. The gas turbine engine component of claim 19, wherein the component comprises at least one of a turbine blade, a turbine vane and a diffuser. 